Social vocalizations of animals have complex frequency and temporal components that require integration in the brain for differentiation. Recent work has shown that combination-sensitive neurons, which are known in the auditory cortex, are also present in the inferior colliculus (IC) of the mustached bat. This finding indicates that selectivity for social vocalizations occurs in the midbrain. The proposal seeks to investigate the neurophysiological response properties of combination-sensitive neurons in the IC of the mustache bat, and investigate whether combination sensitivity contributes to the selectivity. The significance of the research is in gaining and understanding of how these neurons in the midbrain contributes to selectivity of social vocalizations. The study proposes to examine how single and combinations of auditory stimuli affect excitatory, facilitatory and inhibitory responses of single units in three frequency bands. In addition, neural response to fourteen previously identified bat vocalizations are proposed to be tested. Finally, vocalizations that elicit the best neural responses will be separated into their harmonic components and basic acoustic elements. These will be used to assess whether the combination of particular elements contributes to response selectivity. This study will establish the extent of selectivity among social vocalizations in the IC and the contribution of combination sensitivity to selectivity.